In the fields, such as roofing materials and exterior materials of buildings, home appliances and automobiles, the demand for steel sheets having black appearance is increasing from the viewpoints of, for example, design. For blackening the surface of a steel sheet, a method may be employed, which applies a black coating material to the surface of the steel sheet to form a black coating film. In the fields described above, steel sheets with plating, such as hot-dip Zn-plating, hot-dip Al-containing Zn-plating, and hot-dip Al and Mg-containing Zn-plating are used in many cases from the viewpoint of corrosion resistance. These plated steel sheets however have a metallic gloss surface with silver gray color. Accordingly, in order to obtain black appearance of high quality design by application of a black coating material, a thick coating film is required to conceal the color of the plated steel sheet, thereby increasing coating costs. Furthermore, the thick coating film precludes resistance welding, such as spot welding, which poses another disadvantage.
For concealing the metallic gloss and silver gray color of a plated steel sheet without forming of a black coating film, a method for blackening a plating layer itself by oxidizing the same is proposed. PTL 1, for example, discloses a method for forming a oxide layer by bringing a hot-dip Al and Mg-containing Zn-plated steel sheet (hereinafter, also simply referred to as a “plated steel sheet”) into contact with water vapor inside a closed vessel to blacken a hot-dip Al and Mg-containing Zn-plating layer (hereinafter, also simply referred to as a “plating layer”). Hereinafter, bringing a plated steel sheet into contact with water vapor for blackening is also simply referred to as “water vapor treatment” or “treating with water vapor.”
PTL 2 discloses a method for treating plated steel sheets with water vapor by disposing a spacer between the plated steel sheets. The method of PTL 2 is capable of more uniformly blackening of the plating layer surface, as disposition of a spacer between plated steel sheets enables water vapor to contact the plated steel sheets at the periphery and center thereof in the same degree.